Conventionally, inks comprise colorants such as pigment or dye compounds. Pigment based inks, for example, are typically prepared by dispersing pigment agglomerates, formed when individual pigment particles cluster together, in a dispersant. The size of the pigment agglomerates may be reduced by grinding the pigment dispersion using conventional grinding media such as glass, stainless steel, or zirconium oxide.
When preparing pigment dispersions for use in ink-jet inks, for example, conventional grinding media have been found to be unacceptable because they either alter (usually by increasing) the pH of the dispersion to an unacceptable level or result in contamination of the dispersion. An unacceptably high pH can result in inks having incompatibility with ink processing and printing equipment. Contamination can result in, for example, discoloration of inks prepared from the above-described pigment dispersions.
This invention is based on the discovery that plastic media for grinding colorant dispersions, while satisfactorily reducing the colorant agglomerate size in the pigment dispersion, do not unacceptably alter the pH or unduly contaminate the dispersion. Moreover, using plastic media, as described herein, unexpectedly results in an ink composition having a superior color gamut.